1. Field of the Invention
The present invention relates generally to the field of protein chemistry. More particularly, it provides a process of isolating whey proteins. The invention further relates to methods and compositions involving a whey protein isolate that has low turbidity across a wide range of pH values.
2. Description of Related Art
One of the most superior classes of food protein is whey protein. It is known for its excellent amino acid profile, high cysteine content, rapid digestion, and interesting bioactive proteins (lactoglobulins, immunoglobulins, and lactoferrins). Nutritionally speaking, whey protein is known as a naturally complete protein because it contains all of the essential amino acids required in the daily diet. It is also one of the richest sources of branched chain amino acids (BCAAs) which play a large role in muscle protein synthesis. Moreover, some of the individual components of whey protein have been shown to prevent viral and bacterial infection and modulate immunity in animals.
Whey protein makes up one of the two major protein groups in milk, comprising approximately 20% of the total proteins in milk. Caseins account for the other protein portion in milk. In terms of nutrition, whey protein has been rated as a higher quality protein than casein. Additionally, it is more soluble than casein, making it more attractive as food additives. Compositionally, whey protein is actually made up of a mixture of different biological proteins. The majority of whey protein consists of α-lactalbumin (alpha-LB) and β-lactoglobulin (beta-LG). Present in lesser amounts can be immunoglobulin, bovine serum albumin, glycomacropeptide, lactoferrin, and lactoperoxidase.
Whey proteins can be prepared from whey, a by-product of the cheesemaking process, or by running milk through a microfilter and collecting the flow through. Using the cheesemaking process, the production of whey protein involves several steps. After fresh milk is pasteurized, the casein or “curd” is removed to make cheese. The remaining liquid is known as whey. A number of methods are used to isolate and purify whey proteins from liquid whey. These processes include selective precipitation, membrane filtration, and ion exchange chromatography. The two major commercial isolation procedures for whey proteins are ion exchange methods and microfiltration, which yield approximately 90%-95% whey proteins.
In selective precipitation, the whey is physically adjusted to promote insolubility for a particular protein. Proteins tend to aggregate and precipitate at certain pHs and temperature. For example, α-lactalbumin precipitates at a pH of 4.2 and a temperature of 65° C. In contrast, membrane filtration is essentially separating the liquid whey by molecular mass. The liquid whey is run through semi-permeable membranes to separate whey proteins from other components. Both of these processes are volume-dependent, requiring large, expensive equipment to process large volumes of liquid whey.
Purification by ion exchange chromatography can be accomplished through two different processes: selective adsorption or selective elution. In selective adsorption, a single protein is bound to the adsorbent leaving the rest of the whey to run through the exchange column. β-lactoglobulin has been isolated in this way using immobilized retinal. Alternatively, selective elution requires that all the proteins be trapped simultaneously onto an adsorbent. The proteins are washed free from contaminants, and then eluted one by one to manufacture many different purified proteins. As opposed to selective adsorption which isolates only a single protein, selective elution allows the isolation of several different proteins. In comparison to membrane filtration and selective precipitation, ion-exchange chromatography is less volume dependent because capacity depends on mass of protein recovered, not the volume of liquid processed.
Two classes of whey protein are currently in the marketplace: whey protein concentrates and whey protein isolates. Whey protein concentrates are rich in whey proteins, but also contain fat and lactose. Typically, whey protein concentrates are produced by membrane filtration. On the other hand, whey protein isolates consist primarily of whey proteins with minimal fat and lactose. Whey protein isolates usually require a more rigorous separation process such as a combination of microfiltration and ultrafiltration or ion exchange chromatography. After whey proteins are produced in the cheesemaking process from cows' milk and the curd is removed, about 12% of the solids in the remaining byproduct liquid whey is protein. It is generally understood that a whey protein isolate refers to a mixture in which at least 90% of the solids are whey proteins. A whey protein concentrate is understood as having a percentage of whey protein between the initial amount in the byproduct and a whey protein isolate.
Whey proteins in their native state are soluble and have an average isoelectric point (pI) of about 4.6. Two major whey proteins: β-lactoglobulin and α-lactalbumin make up about 90 percent of total whey proteins and have pI values of 4.4 (Etzel 1998) and 5.2 (Hambling 1997), respectively. Whey proteins are known to remain soluble at across a range of pHs. This includes solubility at their isoelectric point, making whey proteins unique in this regard (Damodaran 1996). However, if denatured, whey proteins may aggregate and form precipitates that make the solutions turbid, particularly at and near their isoelectric pH (Damodaran 1996). Exposure to harsh processing conditions is known to denature whey proteins (Damodaran 1996). Most commercial whey protein products form turbid solutions in the pH range of about 4.0 to 5.5.
Although the use of whey protein for human food has demonstrated potential, it has limitations related to the functional properties of the whey proteins, especially in their solubility and turbidity. These shortcomings have prevented whey protein from being properly utilized in the large and growing beverage marketplace. Low-protein, high-carbohydrate drinks, such as soft drinks and fruit juice, encompass nearly 80% of the beverage market. None of these products contain a significant amount of protein. A more nutritional alternative would be a high-protein, reduced sugar drink.
Whey protein isolate would provide an ideal high-quality protein additive for such a beverage. Balancing sweetness and acidity is critical in a beverage formulation. The majority of clear soft drinks are acidic (for example, Coca Cola™ has a 2.5 pH). At a very low pH, the solutions become very tart; consequently, a large quantity of sugar is needed to sweeten the drink. In a drink with high-protein and low sugar content, to achieve the desired sweetness, the target pH would preferably have to be between 3.0 and 4.6. Unfortunately, when added to liquids, whey proteins form sedimentary layers and produce turbid solutions at pHs of 4.0 to 5.5. However, at a pH below 3.0, solutions with whey protein produce clear solutions without precipitation. The problem is that reducing the sugar content of a drink requires a corresponding increase in pH. Consequently, a beverage with high whey protein content at a pH in the desired range would be cloudy and unappetizing to the average consumer. On the other hand, lowering the pH would render the drink clear, but unpalatable unless a substantial amount of sweetener is added. Thus, methods of overcoming the problem of turbid, high pH solutions containing whey protein are needed.